1. Field of the Invention
The present invention relates to, a female thread member used for fastening various components, and to a fastening member using the female thread member.
2. Description of the Related Art
For example, in an assembling process of automobiles, there is widely employed a fastening method in which a female thread member such as a nut is welded to one of components and the other component is fixed by a bolt. At that time, if the bolt is inserted from an inclined direction with respect to an axis of the female thread member, a male thread of the bolt does not properly engage with a female thread of the female thread member. Thus, thread ridges of the male and the female threads bump against each other. In this situation, if the bolt is forcedly rotated by an electric tool or the like, seizing may occur. If such a state occurs, it also becomes difficult to reversely rotate the bolt and a serious problem occurs.
Such seizing caused by inclined insertion occurs at incomplete thread portions formed at thread ridge-leading ends of the bolt and the female thread member. This is because that the incomplete thread portion of the bolt has a smaller diameter than that of its complete thread portion and the incomplete thread portion of the female thread member has a larger diameter than that of its complete thread portion and thus, pitches of the threads of the bolt and the female thread member are deviated from each other and in this state, the thread ridges therebetween can mesh with each other. On the other hand, at the complete thread portions of the bolt and the female thread member, if the pitches thereof are deviated from each other, their thread ridges do not mesh with each other and the seizing is not occurred. From this fact, it is conceivable that the seizing caused by inclined insertion can be prevented by eliminating the incomplete thread portions of the bolt and the female thread member, and by forming the thread ridge-leading ends into a sharply rising shape.
For example, a patent document 1 discloses a female thread member in which a cutting-start portion of the female thread is formed into a stand-up surface and the incomplete thread portion is eliminated. However, if a bolt having no incomplete thread portion is fastened to such a female thread member, it is found that stand-up surfaces of the bolt and the female thread member at thread ridge-leading ends bump against each other depending upon the inclination direction of the bolt with respect to the axis of the female thread member, and the bolt can not smoothly be inserted in some cases.
More specifically, when the stand-up surface at the thread ridge-leading end of the female thread member is located at 12 o'clock position of a clock and the bolt is inclinedly inserted from 12 o'clock position in FIG. 1, the male thread of the bolt first comes into contact with the female thread on the side of 12 o'clock position where the stand-up surface exists. Hence, a flank surface of the male thread of the bolt properly rides on the flank surface of the female thread, and even if the bolt is further rotated as shown with an arrow in FIG. 2, the male thread of the bolt normally threadedly engages with the female thread of the female thread member, and the stand-up surfaces at the thread ridge-leading ends of the bolt and the female thread member do not bump against each other.
On the other hand, when the bolt is inclinedly inserted from a direction of 6 o'clock which is on the side opposite to the stand-up surface, the male thread of the bolt comes into contact with the female thread on the side of 6 o'clock position which is the side opposite to the stand-up surface. Thus, if the bolt is further rotated as shown with an arrow in FIG. 3, the stand-up surfaces between the thread ridge-leading ends of the bolt and the female thread member will bump against each other in accordance with an inclination angle of the bolt. Further, when the bolt is inserted from a direction of 3 o'clock position or 9 o'clock position, there is also a possibility that the stand-up surfaces at the thread ridge-leading ends will bump against each other. Therefore, it is found the problem that the leading end surfaces bump against each other depending upon the inserting direction of the bolt cannot be solved merely by forming the cutting-start portion of the female thread into the stand-up surface and by eliminating the incomplete thread portion.